This invention relates to cleaning and retarding corrosion in heat exchange apparatus. More specifically, it relates to the distribution of liquids which retard corrosion of and remove deposits from tubular air-cooled heat exchangers used in hydrocarbon processing operations.
Prevention of corrosion in equipment used in hydrocarbon refining and processing is and has been the subject of much attention. A specific problem area is the overhead equipment and piping associated with distillation columns in certain processing units. Particular attention to process vapor condensing equipment is necessary. Carbon steels are predominantly used as materials of construction for such equipment. While it is possible to fabricate hydrocarbon processing equipment from metals which are less prone to corrosive attack, such as stainless steel and other high alloys, the cost of such equipment is sufficiently high that it is seldom used. Instead of higher metallurgy, prevention and protection measures are applied to carbon steel equipment. Particularly susceptible to excessive corrosion are the air-cooled heat exchangers used to cool and condense overhead vapors from the columns. Initial cooling of overhead vapors is often accomplished by heat exchange with feedstock or another hydrocarbon stream which requires heating. Further cooling and condensation is normally accomplished in a heat exchanger of the type in which liquid flows through a plurality of tubes over which atmospheric air is passed by means of a fan or fans. Fins are fitted to the exterior surface of the tubes to increase heat transfer area. This type is commonly referred to as air-cooled heat exchangers or fin-fan exchangers. Perhaps the most troublesome process in regard to corrosion is crude fractionation. Exemplary of other processing units of an oil refinery where fin-fan exchangers are subject to excessive corrosion are naphtha stabilizers, catalytic fractionators, catalytic gas plant depropanizers and debutanizers, and deisobutanizers.
The usual method of retarding corrosion in overhead equipment is to contact a corrosion-inhibiting substance with the affected metal surfaces. The corrosion inhibitor is mixed with hydrocarbons withdrawn from the processing unit and the mixture is returned to the unit, usually by injection into the overhead vapor line at a point close to the column. Since corrosion inhibitors are not inexpensive, it is desirable to use them only where the necessity of doing so is proven and then in the minimum quantities necessary for retardation of corrosion. There is often no question but that the use of corrosion inhibitors is necessary, such as in crude fractionation units. However, with many types of feedstocks and hydrocarbon processing units, it is virtually impossible to predict whether corrosion will be a problem. Thus, it is common to install the required equipment and commence use of inhibitors only after routine equipment inspections show that it is necessary. Though much progress has been made in the science of corrosion control in recent years, it is often necessary or expedient to proceed on the basis of initial experience in the processing unit at hand in light of experience in similar processing units. In many respects, the application of corrosion inhibitors may be considered more an art than a science.
In order to be effective, corrosion inhibitors must be brought into contact with the metal surfaces to be protected, so that a film of inhibitor may form over the surface. Inhibitors are generally relatively high boiling materials which are liquids at the conditions of pressure and temperature existing in the protected equipment. Upon introduction into an overhead vapor line, corrosion inhibitor-laden hydrocarbon droplets are spread through the vapor piping and equipment by the flowing stream of vapor. Experience has shown that the upper row or rows of tubes in fin-fan exchangers often are not sufficiently wetted with inhibitor-laden hydrocarbon. The vapor flow path to the upper tubes often contains enough convolutions that the inhibitor-laden droplets separate out and then are re-entrained to flow through the lower tubes but not the upper tubes. Those familiar with the design of liquid separators will readily appreciate the principles involved; put simply, one of the principles is that a liquid droplet often cannot flow along a path including an abrupt change of direction, as a result of its mass and momentum. Also, the mode of vapor distribution is often such that vapor velocities at the upper tubes are too low to keep liquid droplets entrained. The vapor velocity decreases as vapor enters the inlet vapor distribution chamber for the tubes. The velocity is often not high enough to lift inhibitor-laden droplets to the upper tube rows. When the upper tubes are not sufficiently wetted with inhibitor-laden hydrocarbon, the corrosion rate increases. Tube wall thickness is small in comparison with metal thicknesses in other equipment and in the inlet header boxes of fin-fan exchangers. A rate of corrosion acceptable elsewhere in a system may not be acceptable for exchanger tubes. Thus the tubes are particular problem areas. In order to protect the tubes, it is often necessary to add corrosion inhibitor in larger quantities than necessary to protect all of the other surfaces; that is, to attain minimum sufficient protection of exchanger tubes requires an excess for all other surfaces.
Another common problem, which is closely related to the above discussion, is the deposition of ammonium salts, particularly ammonium chloride, on fin-fan exchanger tubes. Such deposition may be referred to as desublimation or reverse sublimation. Compounds present in the hydrocarbon vapor solidify to form salt deposits without passing through a visible liquid state. The salts promote corrosion of the surfaces on which they are deposited. It is common practice to introduce liquid water into the overhead vapor line for the purpose of dissolving and washing away salts as they form on equipment surfaces. Upper tubes of fin-fan exchangers are particularly susceptible to salt accumulation and corrosion resulting therefrom for the same reasons that they are particularly susceptible to corrosion as discussed above; that is, water droplets do not reach the upper tubes. A further problem caused by salt deposits is loss of heat exchange capacity as a result of their impeding the flow of vapor through the upper tubes.
Further background information may be obtained by consulting the U.S. patents mentioned under the heading "Information Disclosure" contained herein. U.S. patents which are exemplary of those disclosing substances used as corrosion inhibitors are U.S. Pat. No. 3,676,327 (Foroulis); U.S. Pat. No. 3,583,901 (Piehl); U.S. Pat. No. 3,537,974 (Foroulis); U.S. Pat. No. 3,516,922 (Anzilotti); U.S. Pat. No. 3,247,094 (Dajani); U.S. Pat. No. 2,920,080 (Thompson); U.S. Pat. No. 2,586,323 (Glassmire and Smith); and U.S. Pat. No. 2,415,161 (Camp).